1. Field of the Invention
The invention relates to a toner for developing electrostatic latent images and the method for produce the same.
2. Related Art
Recently, in the field of image formation by electrophotographic system, it is demanded to raise the compactness and the speediness. As the means for attaining such the objects, miniaturization of the units constituting such as the developing unit of the image forming apparatus is progressed.
For attaining such the miniaturization and speedup of the apparatus, the following properties are required to the constitution of the image forming apparatus such as the developing unit.
(a) The conveying of toner is stabled and the toner is smoothly supplied into the developing device.
(b) Suitable electrical charging amount is maintained by rapid rising up of electrical charging, and toner scattering and fog in non-image area are inhibited.
(c) The charging amount and the developing amount can be constantly and stably maintained with little aging variation such as burying of the external additive of toner since the developer receives strong stirring stress in the developing device by the small developing roller.
(d) The resistivity against heat of the toner is higher than that of the former toner since the cooling mechanism of the image forming apparatus is simplified.
As above-mentioned, the toner is exposed to severe conditions by the miniaturization and speed up of the apparatus. Therefore, a toner capable of forming a suitable image under such the condition is investigated. It has been tried in the investigation to attain the above objects (a) to (d) by improving the external additive.
For example, techniques are known in which inorganic powder having a specified BET surface area value treated by a silane coupling agent or silicone oil is used as the external additive, a planar fine particle is used as the external additive, or a substance having a chain or branched structure constituted by covalent bonded 6 to 500 fine particles is uses as the external additive.
However, the properties demanded by the inventors are not satisfied by the above external additives.
On the other hand, exact reproduction of digital images is required. For satisfying such the needs, miniaturize of the toner particle is investigated and a polymerization toner seems most suitable for miniaturized toner particle since the particle diameter of the polymerization toner can be controlled in the production process. It has been tried to attain high speed charging up by the small size developing unit by adding an external additive to the polymerization toner. However, a problem is posed that the external additive is easily released from the toner particle surface when the external stress such as that caused by stirring is applied since the adhering force of the polymerization toner particle with the external additive is weak.
Therefore, an external additive difficultly released from the polymerization toner particle surface has been investigated. However, one capable of fitting for the use in a small size image forming apparatus cannot be found yet, in which a large stress is applied on the occasion on the image formation.
Moreover, the electrophotographic system becomes to competed with light pressing work accompanied with the speedup and network formation thereof. Consequently, the formation of an image with a high resolving power and image quality is required. For satisfying such the requirements, it is tried to attain the high resolving power by a toner having small particle diameter.
The increasing in the relative surface area of the toner accompanied with the decreasing in the particle diameter causes a problem that the charging amount per init area is considerably increased and the stability of the developing amount is influenced.
And then the developing ability of the toner becomes instable, so that the high resolving power cannot be obtained even though the diameter of the toner particle is made small. Therefore, it is very difficult to output a high quality toner image. On such the background, techniques by improvement of the external additive such as the use of needle-like shaped titania or titania-including silica and techniques noting on the transferring ability and the resolving power are proposed.
However, these techniques are insufficient yet from the viewpoint of higher resolving power and stability of image expected by the inventors.
Besides, the technique for the polymerization toner suitable for making small the diameter of toner is considerably progressed, in which a technique of emulsion association is noted. One of the reasons of that is that the shape of the toner particle can be easily controlled and the particle diameter distribution of the toner can be controlled so as to be considerably sharp compared with the usual toner particle.
Furthermore, techniques in which resin particles are fixed on the toner particle or resin layer is provided on the toner particle surface have been proposed for making uniform the charging on the toner particle.
However, an image having high resolving power is difficultly obtained by the above techniques and expected effects are not always obtained even when the improvement of the external additive is applied in combination.
The polymerization toner causes the problem that the adhering and fixing strength with the external additive is weak and the external additive is easily released from the toner particle surface. As the reason of that, it is supposed that the toner particle produced by the polymerization method cannot strongly trap the external additive since such the particle has no corner and the surface of it is smooth.
According to such the background, an external additive which does not release from the toner particle surface and a toner stable in the charging amount, developing amount and transferring ability and constantly giving high resolving power are required.
The invention is attained on the above-mentioned back ground.
On a first aspect of the invention, an object of the invention is to provide a toner by which suitable image formation can be performed by an image forming apparatus corresponding to miniaturization and speedup. Namely, an first object of the invention is to provide a toner for developing electrostatic images employable for high speed image formation, which can be smoothly conveyed and electrically charged rapidly when the toner is supplied in the developing device.
On a second aspect of the invention, a second object of the invention is to provide a toner for developing an electrostatic image having durability so that the image formation can be performed stably without releasing of the external additive form the toner particle surface even when large stress is applied to the toner by stirring in a miniaturized developing device or burying the external additive into the toner particle.
On a third aspect of the invention, a third object of the invention is to provide a toner having high heat resistive stability so that the image formation can be stably performed in an apparatus in which the cooling mechanism of the apparatus is simplified.
On a fourth side of the invention, an object is to provide a toner for developing electrostatic images using an external additive suitable for a polymerized small diameter toner by which the electrical charging amount, the developing amount and the transferring ability are stabilized and high resolving power of image can be obtained.